Optical filters including optical ring resonators are known. In such optical filters, a waveguide in a closed loop or ring is coupled to one or more input/output waveguides. If the circumference of the looped waveguide equals an integral number of wavelengths (resonant wavelengths) of incoming light supplied on the input waveguide, and the coupling ratio is relatively low, the intensity of the incoming light will increase over multiple passes around the ring due to constructive interference. Light at the resonant wavelength can then be output on the output waveguide through optical coupling between the looped waveguide and the output waveguide. Light at other wavelengths, however, will be suppressed in the looped waveguide due to destructive interference.
Conventional optical ring resonators may also include a heater to thermally vary the refractive index and thus the optical path length around the looped waveguide. By changing the optical path of the looped waveguide, the resonant wavelength may also be changed, thereby allowing the optical ring resonator to act as a tunable filter.
Multiple waveguide rings can be coupled to one another to provide a filter having a passband over a particular range of wavelengths. The spectral width of the passband of the filter depends on the amount of optical coupling or coupling ratio between adjacent rings. Since ring dimensions (e.g., the circumference and waveguide width) are substantially fixed, the spectral width of the passband, i.e., the filter bandwidth, is also fixed, even though the center wavelength of the passband may be thermally tuned.
Accordingly, a filter having both a center wavelength and passband that is tunable would be desirable.